The present invention relates generally to the field of millimeter wave amplifiers and, more particularly, to those devices utilizing negative resistance properties to obtain reflection gain in reduced height waveguides. Previous millimeter wave amplifiers utilizing such devices as Gunn diodes are either narrowband or utilize a coaxial cavity mounted diode which is connected to the waveguide by iris coupling through the coaxial wall. Associated with the diode and its package are certain parasitic reactances such as diode capacity, inductive leads, etc. In order to obtain the maximum bandwidth, it is necessary to provide additional reactive elements to at least partially cancel the effects of the diode parasitics. Previously, these reactive elements took the form of circuit elements such as tuning slugs, located wavelengths or fractional wavelengths from the diode.